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CEACCP > Monitoring of Neuromuscular Block > Flashcards

Monitoring of Neuromuscular Block Flashcards

1
Q

Does TOF stimulation require a control twitch?

A

No

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2
Q

What kind of block is fade a feature of?

A

Non-depolarizing only

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3
Q

What should the TOF count be greater than to ensure reversibility?

A

3

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4
Q

How frequently can you perform a TOF stimulation?

A

It shouldn’t be performed any more frequently than once every 10 seconds, otherwise it can lead to recovery from block in the stimulated muscle

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5
Q

What kind of neuromuscular monitoring is accelomyography best suited to?

A

TOF and PTC monitoring

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6
Q

What must the TOF ratio be fore acceptable recovery?

A

>0.9

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7
Q

Which is more sensitive, DBS or TOF?

A

DBS is easier to assess clinically than TOF so is more sensitive

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8
Q

Which muscle is more accurate in reflecting block recovery, the adductor pollicus or the orbicularis oculi?

A

The adductor pollicis

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9
Q

Which recovers from block faster, the diaphragm or the adductor pollicis?

A

The diaphragm

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10
Q

Can you monitor DBS with acceloromyography?

A

DBS cannot be reliably monitored using acceleromyography

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11
Q

What is a potential source of error with EMG recording?

A

EMG recording may detect direct muscle stimulation.

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12
Q

What is the amplitude of an evoked EMG potential proportional to?

A

The amplitude of an evoked EMG potential is proportional to the number of neuromuscular junctions stimulated.

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13
Q

What preload is advised to be placed on the adductor pollicis muscle when using MMG monitoring?

A

200-300 grams

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14
Q

What is the resistance of skin in ohms?

A

0 - 5 kOhms

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15
Q

What disease states produce increased skin resistance?

A

Diabetes mellitus Hypothyroidism (thick skin)

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16
Q

How does temperature affect skin resistance?

A

Cold increases resistance Heat decreases resistance

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17
Q

Will skin resistance be affected by the type of electrode used?

A

Yes, the surface area and quality of contact varies with different electrodes

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18
Q

What does DBS consist of?

A

2 bursts of tetanus at 50Hz Each impulse separated by 20ms and the 2 bursts separated by 750ms

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19
Q

What is profound block best assessed by?

A

PTC

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20
Q

What issues can residual neuromuscular block cause?

A
  • impairs ventilatory response to hypoxia
  • increased risk of aspiration
  • airway obstruction
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21
Q

What criteria must a nerve have to be used to assess degree of neuromuscular block?

A
  • must have a motor element
  • must be close to the skin
  • contraction in the muscle/muscle group which the nerve supplies must be visible or accessible to evoked response monitoring
22
Q

What current needs to be applied to generate a response through all the nerve fibres and hence make a maximal muscle contraction?

A
  • current for 0.1 - 0.3 ms
  • usually 25% above the maximal stimulus (supramaximal)
23
Q

What properties would the ideal nerve stimulator have?

A
  • battery operated
  • able to deliver a constant current up to 80 mA
  • adequacy of the electrical contact should be displayed on the monitor screen
  • pulse stimulus should last no more than 0.3 ms and be monophasic, square wave type to ensure constant current throughout the stimulus
  • polarity of electrode leads should be indicated
  • should be able to deliver a variety of patterns - single twitch 1Hz, TOF 2Hz with 10s between trains, tetanic at 50Hz for up to 5s and DBS
  • be able to monitor evoked responses
24
Q

How does single twitch stimulation work?

A
  • needs a control twitch before giving neuromuscular blocker
  • single square wave supramaximal stimulus is applied to a peripheral nerve for 0.2 ms at regular intervals and the evoked response is observed
  • twitch will be depressed when neuromuscular blocking agent occupies >75% of the post-synaptic nicotinic receptors
  • twitch depression must be >90% to provide good conditions for abdo surgery
  • useful at the onset of neuromuscular block
  • can also be used in the post-tetanic count but don’t need a control twitch height for that
25
Q

How does TOF stimulation work?

A
  • freq 2Hz (4 stimuli separated by 0.5 s)
  • then repeated every 10s (train freq 0.1 Hz)
  • can compare T1 to T4 (TOF ratio)
  • non-depolarizing agents produce fade, T4 reduces in amplitude, followed by T3, 2 and 1
26
Q

At what % of depression of T1 does T4 disappear at?

A

T4 disappears at 75% depression of T1

27
Q

At what % depression of T1 do T3 and T2 disappear at?

A

T3 disappears at 80-85% depression of T1

T2 disappears at 90% depression of T1

28
Q

How many twitches should be present for reversal of residual neuromuscular block?

A

TOF count of 3 or more

29
Q

What TOF ratio should be achieved before tracheal extubation?

A

0.9

30
Q

What is phase 2 block?

A

If you give repeated doses or an infusion of succinylcholine then you will get features of a non-depolarising block such as fade

31
Q

What is tetanic stimulation?

A
  • uses high freq (50-200 Hz) with a supramaximal stimulus for a set time (5 seconds)
  • the response is a tetanic contraction
  • after giving non-depolarizing NMB sustained muscular contraction won’t be possible due to fade - which corresponds with degree of NM block
  • very sensitive
  • can elicit minor degrees of NM block
  • limited use as extremely painful
32
Q

What is the response to tetanic stimulation in a partial depolarizing block?

A

Fade is not observed.

The amplitude of evoked response will be lower but tetanic contraction is maintained

33
Q

What is a post tetanic count?

A
  • during profound NDNMB there may be no response to TOF or single twitch stimulation
  • if a 5s 50Hz tetanic stimulus is administered, after no twitch response has been elicited, followed 3 s later by a further single twitch at 1 Hz there may be a response
  • this will be seen before TOF reappears
34
Q

What is post-tetanic facilitation?

A
  • on completion of tetanic stimulus, ACh synthesis and mobilization continues for a short time
  • as a result there is an increased store of ACh which causes an enhanced response to subsequent single twitch stimulation
  • the number of post-tetanic twitches is an indication of when the first twitch of the TOF will reappear
    • the first twitch will normally return with a PTC of 9 when using atracurium or vecuronium
35
Q

When is PTC useful?

A

When profound neuromuscular block is required, for example during retinal surgery, when movement or coughing could have devastating effects

36
Q

How long should you wait between tetanic stimulation?

A

6 minutes minimum.

Because if 2 PTCs are administered in quick succession the degree of NMB will be underestimated.

37
Q

What is DBS?

A
  • developed to detect even small degrees of NMB
  • fade with DBS is easier to appreciate clinically than with TOF
  • 2 short bursts of tetanus at 50Hz at supramaximal current
  • each burst lasts 0.2 ms, separated by 20 ms with two bursts separated by 750ms
  • if NDNMB is used - the response to second burst is reduced
  • the ratio is the DBS ratio
38
Q

What is mechanomyography (MMG)?

A
  • measures evoked muscle tension
  • most commonly studied is adductor pollicis in the thumb
  • if the thumb is stabilized and placed under a fixed amount of tension (preload) then evoked responses can be measured as a change in tension develops
  • uses a strain gauge transducer and recorder
  • thumb doesn’t move - contraction is isometric
  • this is the gold standard for assessing any nerve stimulation
  • disadv - cumbersome, impractical for theatre
39
Q

What is electromyography (EMG)?

A
  • adductor pollicis or ulnar nerve most commonly used
  • records a compound AP that occurs during muscular contraction
  • evoked APs are a measurement of electrical changes that occur in muscle during stimulation - these are the equivalent to the muscular contraction that occurs after excitation-contraction coupling
  • requires 3 electrodes
  • creates a number of low voltage motor APs - summated into a compound AP which must be amplified
40
Q

What are the disadvantages of EMG?

A
  • prone to interference esp from diathermy
  • hand temp/movement will interfere
  • prone to drift
  • another potential source of inaccuracy is direct muscle stimulation
41
Q

What is acceleromyography?

A
  • similar principle to MMG, but instead of measuring force of contraction directly, acceleration of the contracting muscle is measured
  • force calculated using Newton’s second law of motion ( force = mass x acceleration)
  • acceleration is measured using a piezoelectric ceramic wafer strapped to the thumb - produces a voltage proportional to acceleration when moves
  • then converted to electrical signal and displayed as twitch response
  • PTC can also be measured this way, but not DBS or tetanus
42
Q

Which muscles is onset and offset of a block faster in?

A

Central muscles with good blood supply

eg diaphragm and larynx

Conversely those with poor blood supply (peripheral muscles) eg adductor pollicis

43
Q

How do the muscles of the upper airway and pharynx behave at the onset of neuromuscular block?

A

At onset they behave as central muscles so the block works faster, but are sensitive to NMBs and recovery is slow, mirroring peripheral muscles

44
Q

Why is the orbicularis oculi the ideal muscle to monitor at induction of anaesthesia and tracheal intubation?

A
  • it’s similar to a central muscle
  • onset of block will be similar to laryngeal muscles and diaphragm
  • single twitch/TOF is most valuble at induction
    • disappearance of TOF corresponds to optimal intubating conditions
45
Q

What conditions are neuromuscular monitoring essential in?

A
  • after prolonged infusions of NMB drugs or when long-acting drugs are used
  • when surgery/anaesthesia is prolonged
  • when inadequate reversal may have devastating effects, eg severe resp disease/morbid obesity
  • when giving reversal may cause harm (eg tachyarrhythmias/cardiac failure)
  • liver/renal dysfunction where pharmacokinetics of muscle relaxants may be altered
  • neuromuscular disorders such as myaesthenia gravis or Eaton-Lambert syndrome
46
Q

What preload should be used in MMG monitoring?

A

200-300g

47
Q

What is the amplitude of the evoked EMG potential proportional to?

A

To the number of neuromuscular junctions stimulated

48
Q

What does skin resistance vary from?

A

0 - 5 kOhms

49
Q

What is profound block best detected by?

A

PTC is most accurate for this

50
Q
A

CEACCP (16 decks)

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